Methods and systems for fine timing synchronization

ABSTRACT

A method for determining timing synchronization for demodulating a signal by a receiver, comprises the steps of: generating a channel response for the signal; transforming the signal into the time domain using an inverse fast fourier transform (“IFFT”); determining a signal power for the transformed signal as a function of the generated channel response; and calculating the timing synchronization by the receiver as a function of the determined signal power.

FIELD OF INVENTION

This invention relates to methods and systems for channel estimation fordemodulating an orthogonal frequency division multiplexing (“OFDM”)signal, and, in particular, to methods and systems for timingsynchronization for an OFDM signal.

BACKGROUND

Orthogonal frequency division multiplexing system is a multi-carriertransmission technique that uses orthogonal subcarriers to transmitinformation within an available spectrum. Since the subcarriers areorthogonal to one another, the subcarriers can be spaced much moreclosely together within an available spectrum than, for example, theindividual channels in a conventional frequency division multiplexing(“FDM”) system. Many modern digital communications systems are turningto the OFDM system as a modulation scheme for signals that need tosurvive in environments having multipath-propagation or stronginterference, including the IEEE 802.11a standard, the Digital VideoBroadcasting Terrestrial (“DVB-T”) standard, the Digital VideoBroadcasting Handheld (“DVB-H”) standard, the Digital Audio Broadcast(“DAB”) standard, and the Digital Television Broadcast (“T-DMB”)standard.

In an OFDM system, the subcarriers may be modulated with a low-rate datastream before transmission. It is advantageous to transmit a number oflow-rate data streams in parallel instead of a single high-rate streamsince low symbol-rate schemes suffer less inter-symbol interference(“ISI”) caused by multipath.

OFDM modulated signals can be transmitted in transmission frames, whereeach transmission frame consists of a number of symbols. The receptionof these signals depends on successful acquisition of symbol timing andframe timing. Symbol timing acquisition can be accomplished by findingthe boundary of each symbol; whereas frame timing acquisition can beaccomplished by finding the starting symbol of each transmission frame.

In particular, with respect to OFDM modulated signals, timingsynchronization and frequency synchronization are difficult. It isdifficult to exactly synchronize symbols between the transmitter and thereceiver. Timing synchronization requires that the beginning of eachOFDM symbol be determined within each frame. Unless the correct timingis known, the receiver cannot remove cyclic prefixes at the correcttiming instance. Thus, individual symbols cannot be correctly separatedbefore a Fast Fourier Transform (“FFT”) is applied to demodulate thesignal.

In a wireless environment with multipath reception, finding the optimalFFT window timing can result in the lowest inter-symbol-interference(“ISI”), and therefore the best receiver performance. Fine timingsynchronization serves this purpose. To find the timing window, i.e.,the start of an FFT window, a conventional method is to locate thestrongest path via a time domain correlation or inverse FFT and thenplace the FFT window a few samples shift from the strongest path. In anadditive white Gaussian noise (“AWGN”) environment, this scheme worksjust fine since the delay is very limited. However, in a dynamicenvironment with various multipath delays and multipath profiles, suchmethods are not adequate. Therefore, it is desirable to provide methodsfor timing synchronization for OFDM modulated signals.

SUMMARY OF INVENTION

An object of this invention is to provide methods and systems for finetiming synchronization to aid in processing a received signal that canaccount for multipath delays.

Another object of this invention is to provide methods and systems forfine timing synchronization to aid in processing a received signal thatcan account for various signal profiles.

Yet another object of this invention is to provide methods and systemsfor fine timing synchronization for a signal having improvedperformance.

Briefly, the present invention discloses methods for determining timingsynchronization for demodulating a signal by a receiver, comprising thesteps of: generating a channel response for the signal; transforming thesignal into the time domain using an inverse fast fourier transform(“IFFT”); determining a signal power for the transformed signal as afunction of the generated channel response; and calculating the timingsynchronization by the receiver as a function of the determined signalpower.

An advantage of this invention is that methods and systems for finetiming synchronization are provided to aid in processing a receivedsignal that can account for multipath delays.

Another advantage of this invention is that methods and systems for finetiming synchronization are provided to aid in processing a receivedsignal that can account for various signal profiles.

Yet another advantage of this invention is that methods and systems forfine timing synchronization for a signal having improved performance areprovided.

DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, and advantages of theinvention can be better understood from the following detaileddescription of the preferred embodiment of the invention when taken inconjunction with the accompanying drawings in which:

FIG. 1 illustrates an OFDM frame having a cyclic prefix and a body.

FIG. 2 illustrates a block diagram of a communications system of thepresent invention.

FIG. 3 illustrates a block diagram of a fine timing synchronizationblock of the present invention.

FIG. 4 illustrates a block diagram of a channel response block of thepresent invention.

FIG. 5 illustrates a diagram in which optimal timing for a FFT window isfound for a frame of a signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the embodiments, reference ismade to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration of specific embodiments in whichthe present invention may be practiced.

FIG. 1 illustrates an OFDM frame having a cyclic prefix (“CP”) and abody. A symbol 10 can have a CP 12 having a length of Ncp points and abody 14 having a length of N points. The CP 12 is a copy of a latterportion 16 of the body 14 of the symbol 10 that is appended to thebeginning of the symbol body 14. The CP 12 can serve as a buffer toavoid inter-symbol interference. Typically, the CP 12 is discarded whendecoding the symbol 10. For instance, the CP 12 should be removed beforeapplying FFT demodulation during the decoding of the symbol 10.

FIG. 2 illustrates a block diagram of a communications system of thepresent invention. A signal is inputted to a transmitter 20 fortransmission over a channel 22, e.g., over-the-air wireless channel. Thetransmission can be received by a receiver 24 for processing anddecoding.

The transmitter 24 comprises a serial-to-parallel converter 26, aninverse fast Fourier transform (“IFFT”) block 28, a parallel-to-serialconverter 30, and a CP adder block 32. The transmitter 24 may alsocomprise other blocks for transmitting the signal over the channel 22.However, to aid in the understanding of the invention, the above listedblocks are used to illustrate several key blocks of the transmitter 24.It is understood by a person having ordinary skill in the art that atransmitter (or receiver) of the present invention can have other blocksfor transmitting or receiving the signal.

The receiver 24 can comprise a digital front end block 42, a CP removalblock 40, a Fast Fourier Transform (“FFT”) block 38, a channel estimator36, a decoder 34, a fine timing synchronization block 44, and a coarsetiming synchronization block 46. The received signal from the channel 22can be processed by the receiver 24 by first being processed by thedigital front end block 42. The digital front end block 42 processes thereceived signal from an analog signal to a digital signal yy having apredefined sampling rate. The digital signal yy can be outputted to theCP removal block 40 and the coarse timing synchronization block 46. Thecoarse timing synchronization block 46 can estimate various errors ofthe digital signal yy and make any corrections as necessary. Inparticular, the coarse timing synchronization block 46 can estimate thecoarse timing for the receiver 24. The coarse timing can provide thereceiver 24 with useful timing information for applying a first FFT onthe received signal.

The CP removal block 40 receives the digital signal yy, a coarse timingsynchronization, and a fine timing synchronization to accurately removethe CP from the digital signal yy. The CP removal block 40 outputs thesignal y (that is the digital signal without the CP) to the FFT block38. The FFT block 38 performs a FFT operation on the signal y to covertthe signal y from the time domain signal to a frequency domain signal Y.The frequency domain signal is outputted to the channel estimator 36.The channel estimator 36 performs channel estimation on the signal Y togenerate a channel frequency response H to generate the estimatedsymbols X_(est). The estimated symbols X_(est) are outputted to thedecoder 34 for further processing.

The frequency domain signal Y and the estimated symbol X_(est) areinputted to the fine timing synchronization block 44 for generating afine timing value. The fine timing value provides the correct startingposition of the FFT window for the signal to remove the CP. Thus, the CPremoval block 40 can use the fine timing value to accurately delete theCP from the signal y.

FIG. 3 illustrates a block diagram of a fine timing synchronizationblock of the present invention. The fine timing synchronization block 44of the present invention comprises a channel estimator 62, an IFFT block64, a signal power calculation block 66, a path threshold setter 70, anda fine timing synchronization block 68. The channel estimator 62receives the signal Y(k) in the frequency domain and the estimatedsymbols X_(est)(k) for the signal to generate an estimated channelresponse H_(est)(k) in the frequency domain, where k is the frequencycarrier number. The estimated channel response H_(est)(k) can be foundby the following equation:H _(est)(k)=Y(k)/X _(est)(k),  Equation [1]where k is the frequency carrier number.

The IFFT block 64 receives the estimated channel response H_(est)(k) andapplies an IFFT on the received channel response H_(est)(k) to convertthe channel response to the time domain, h_(est)(n), wherein n is theindex number. The channel response, h_(est)(n), in the time domain isthen inputted to the signal power calculation block 66 to calculate thepower of the signal P_(h)(n), also referred to as the signal power. Thesignal power P_(h)(n) can be calculated by the following equation:

$\begin{matrix}{{P_{h}(n)} = \lbrack {{{abs}( {h( {{mod}( {{n - \frac{N}{2}},N} )} )} \rbrack}^{2},} } & {{Equation}\mspace{14mu}\lbrack 2\rbrack}\end{matrix}$where mod(a,b) is the modulo operator and N is a FFT length. The signalpower P_(h)(n) is inputted to the path threshold setter 70 and the finetiming synchronization block 68.

The path threshold setter 70 sets values of the signal power P_(h)(n)below a predefined threshold to zero. This is done to eliminate possiblenoise from being introduced into the calculation for fine timingsynchronization. Thereby, only the signal power values above a certainthreshold are used for the fine timing synchronization block 68. Thefiltered signal power can be denoted by the following equation:

$\begin{matrix}{{\hat{P_{h}}(n)} = \{ \begin{matrix}{{P_{h}(n)},} & {{{for}\mspace{14mu}{P_{h}(n)}} \geq {{path}\mspace{14mu}{threshold}}} \\{0,} & {{{for}\mspace{14mu}{P_{h}(n)}} < {{path}\mspace{14mu}{threshold}}}\end{matrix} } & {{Equation}\mspace{14mu}\lbrack 3\rbrack}\end{matrix}$The fine timing synchronization block 68 uses the signal power{circumflex over (P)}{circumflex over (P_(h))} (n) to calculate a finetiming value to indicate the start of the FFT window for demodulating ofthe signal.

The fine timing for the FFT window can equal the following:

$\begin{matrix}{{{fine}\mspace{14mu}{timing}} = {\arg\;{\min_{idx}{{abs}( {( {\sum\limits_{r = 0}^{{idx} - 1}{\hat{P_{h}}(r)}} ) - ( {\sum\limits_{r = {{idx} + {Ncp}}}^{N - 1}{\hat{P_{h}}(r)}} )} )}}}} & {{Equation}\mspace{14mu}\lbrack 4\rbrack}\end{matrix}$where min is the minimum function, abs is an absolute value function,idx is an index, N is a body length of a frame of a signal, and Ncp is acyclic prefix length of the frame of a signal, and {circumflex over(P)}{circumflex over (P_(h))} (n) is the filtered signal power. Theindex idx can start from a maximum index, in which a maximum power valuefor the determined signal power is located at the maximum index. Also,the index idx may start at other values until a minimum is found for theminimum function in Equation [4].

FIG. 4 illustrates a block diagram of a channel response block of thepresent invention. The channel response block 62 of the presentinvention comprises a divider unit 72 and a slicer 74. The signal Y(k)and a hard decision symbol X_(dec) are inputted to the channel estimator62. The slicer 74 receives an estimated symbol X_(est), which isdemapped to generate the hard decision symbol X_(dec). The signal Y(k)and the hard decision symbol X_(dec) are inputted to the divider unit72. The divider unit 72 divides the signal Y(k) by the hard decisionsymbol X_(dec) to determine an estimated channel response H_(est)(k).

FIG. 5 illustrates a diagram in which optimal timing for a FFT window isshown for a frame of a signal. A frame 80 of the signal can start at anindex labeled n=0 and end at the index n=N−1, where N is the FFT length.The minimum value of the absolute difference of the following two items:

(1) the sum of the signal powers P_(h)(n) for n=0, 1, 2, . . .optimal_timing, in a pre-path 82; and

(2) the sum of the signal powers P_(h)(n) for n= . . . N−2, N−1, in apost-path 84, can be used to determine an optimal timing index for theFFT window. The optimal timing index is when n=optimal_timing. The indexof the post-path can start at the optimal timing plus the cyclic prefixlength Ncp. Furthermore, the optimal timing minus N/2 can signify thestart of the FFT window since there is an N/2 shift from Equation [2].

While the present invention has been described with reference to certainpreferred embodiments or methods, it is to be understood that thepresent invention is not limited to such specific embodiments ormethods. Rather, it is the inventor's contention that the invention beunderstood and construed in its broadest meaning as reflected by thefollowing claims. Thus, these claims are to be understood asincorporating not only the preferred apparatuses, methods, and systemsdescribed herein, but all those other and further alterations andmodifications as would be apparent to those of ordinary skilled in theart.

We claim:
 1. A method for determining timing synchronization fordemodulating a signal by a receiver, comprising the steps of: generatinga channel response for the signal; transforming the signal into the timedomain using an inverse fast fourier transform (“IFFT”); determining asignal power for the transformed signal as a function of the generatedchannel response; and calculating the timing synchronization by thereceiver as a function of the determined signal power, comprising thesubsteps of: determining a first summation value for a first selectedrange of the determined signal power; determining a second summationvalue for a second selected range of the determined signal power; anddetermining a minimum index for the signal as a function of the firstsummation value and the second summation value, wherein, at the minimumindex, the absolute value of the difference between the first summationvalue and the second summation value is minimized and is used as thetiming synchronization.
 2. The method of claim 1 wherein the generatedchannel response is a function of the signal and an estimated symbol forthe signal.
 3. The method of claim 1 wherein a maximum index isdetermined for the signal, wherein a maximum power value for thedetermined signal power is located at the maximum index, wherein thesignal has an index reference starting from zero to N, wherein the firstselected range of the first summation value starts from zero to themaximum index minus one, and wherein the second selected range of thesecond summation value starts from the maximum index plus a cyclicprefix length to N−1.
 4. The method of claim 3 wherein the minimum indexis equal to${\min_{idx}{{abs}( {( {\sum\limits_{r = 0}^{{idx} - 1}{\hat{P_{h}}(r)}} ) - ( {\sum\limits_{r = {{idx} + {Ncp}}}^{N - 1}{\hat{P_{h}}(r)}} )} )}},$wherein idx is an index of the signal, the cyclic prefix length isN_(cp), and P_(h)(r) is the determined signal power as a function of theindex of the signal.
 5. The method of claim 1 wherein, after thedetermining step and before the calculating step, comprising the step:setting the determined signal power to zero for signal power valuesbelow a predefined threshold.
 6. A method for determining timingsynchronization for demodulating a signal by a receiver, comprising thesteps of: generating a channel response for the signal, wherein thegenerated channel response is a function of the signal and an estimatedsymbol for the signal; transforming the signal into the time domainusing an inverse fast fourier transform (“IFFT”); determining a signalpower for the transformed signal as a function of the generated channelresponse; setting the determined signal power to zero for signal powervalues below a predefined threshold; and calculating the timingsynchronization by the receiver as a function of the determined signalpower, comprising the substeps of: determining a first summation valuefor a first selected range of the determined signal power; determining asecond summation value for a second selected range of the determinedsignal power; and determining a minimum index for the signal as afunction of the first summation value and the second summation value,wherein, at the minimum index, the absolute value of the differencebetween the first summation value and the second summation value isminimized and is used as the timing synchronization.
 7. The method ofclaim 6 wherein a maximum index is determined for the signal, wherein amaximum power value for the determined signal power is located at themaximum index, wherein the signal has an index reference starting fromzero to N, wherein the first selected range of the first summation valuestarts from zero to the maximum index minus one, and wherein the secondselected range of the second summation value starts from the maximumindex plus a cyclic prefix length to N−1.
 8. The method of claim 6wherein the minimum index is equal to${\min_{idx}{{abs}( {( {\sum\limits_{r = 0}^{{idx} - 1}{\hat{P_{h}}(r)}} ) - ( {\sum\limits_{r = {{idx} + {Ncp}}}^{N - 1}{\hat{P_{h}}(r)}} )} )}},$wherein idx is an index of the signal, the cyclic prefix length isN_(cp), and P_(h)(r) is the determined signal power as a function of theindex of the signal.
 9. A method for determining timing synchronizationfor demodulating a signal by a receiver, comprising the steps of:generating a channel response for the signal, wherein the generatedchannel response is a function of the signal and an estimated symbol forthe signal; transforming the signal into the time domain using aninverse fast fourier transform (“IFFT”); determining a signal power forthe transformed signal as a function of the generated channel response;setting the determined signal power to zero for signal power valuesbelow a predefined threshold; and calculating the timing synchronizationby the receiver as a function of the determined signal power, comprisingthe steps: determining a first summation value for a first selectedrange of the determined signal power; determining a second summationvalue for a second selected range of the determined signal power; anddetermining a minimum index for the signal as a function of the firstsummation value and the second summation value, wherein, at the minimumindex, the absolute value of the difference between the first summationvalue and the second summation value is minimized and is used as thetiming synchronization, wherein the minimum index is equal to${\min_{idx}{{abs}( {( {\sum\limits_{r = 0}^{{idx} - 1}{\hat{P_{h}}(r)}} ) - ( {\sum\limits_{r = {{idx} + {Ncp}}}^{N - 1}{\hat{P_{h}}(r)}} )} )}},$and wherein idx is an index of the signal, the cyclic prefix length isN_(cp), and P_(h)(r) is the determined signal power as a function of theindex of the signal.
 10. The method of claim 9 wherein a maximum indexis determined for the signal, wherein a maximum power value for thedetermined signal power is located at the maximum index, wherein thesignal has an index reference starting from zero to N, wherein the firstselected range of the first summation value starts from zero to themaximum index minus one, and wherein the second selected range of thesecond summation value starts from the maximum index plus a cyclicprefix length to N−1.